Standard sand blasting is a procedure whereby hard particulate matter, typically fine sand, is suspended in a stream of air which is directed against a surface to be eroded by the sand. The stream moves along a hose to a nozzle provided at its outlet end with a restriction that raises the speed of the stream while lowering its static pressure. Thus the rapidly moving sand can hit the surface being treated with considerable force so such a procedure can remove rust, paint, and similar abradable substances.
In the standard system, dry sand blasting, essentially moisture-free sand and air only are used. Wet sand cannot be used because it cannot be entrained properly by the air stream, but instead will clump and clog the equipment. The problem with this system is that it generates a great deal of dust. Not only is the sand in the stream quite dusty, but it raises a great deal of dust in the form of the material abraded from the surface being sand blasted. In fact it is necessary to provide the operators of such equipment with respirators so they do not breath this dust, and it is necessary to enclose the environs of the job, which can be quite difficult for instance when a building exterior is being sand blasted from scaffolding.
Hence the technique of wet sand blasting as described in German patent No. 2,724,318 has been developed. In the standard form an aspiration line provided with a valve has one end in a water reservoir and another end opening into the nozzle of the machine right at the restriction of its outlet end. The low static pressure at the restriction sucks the water out of the reservoir like in a paint sprayer. This water moistens the sand so that it does not generate dust itself, and it in turn moistens the particles it knocks loose to prevent them from becoming airborne.
It is, however, fairly difficult to adjust the feed rate for the water or water/air suspension with such a system. If the mixture is too lean, that is with too little water, the outer particles in the spray wil be adequately wetted but the inner ones will not. If adjusted to wet these inner particles in the jet, so much water is projected against the surface that it creates runoff which is normally not permissible. This adjustment problem is present no matter where the water is introduced into the stream.